Petrogenesis of the Majiari ophiolite (western Tibet, China): Implications for intra-oceanic subduction in the Bangong-Nujiang Tethys

The Majiari ophiolite lies in the western Bangong-Nujiang Suture Zone, which separates the Qiangtang and Lhasa blocks in central Tibet. The ophiolite consists of peridotite, gabbro/diabase and basalt. Zircon U-Pb dating yielded an age of 170.5 +/- 1.7 Ma for the gabbro, whereas 40Ar/39Ar dating of plagioclase from the same gabbro yielded ages of 108.4 +/- 2.6 Ma (plateau age) and 112 +/- 2 Ma (isochron age), indicating that the ophiolite was formed during the Middle Jurassic and was probably emplaced during the Early Cretaceous. Zircons from the gabbro have epsilon(Hf)(t) values ranging from +6.9 to + 10.6 and f((Lu/Hf)) values ranging from -0.92 to -0.98. Mafic lavas plot in the tholeiitic basalt field but are depleted in Nb, Ta and Ti and enriched in Rb, Ba and Th in the NMORB-normalized trace element spider diagram. These lavas have whole-rock epsilon(Nd)(t) values of + 5.9 to + 6.6, suggesting that they were derived from a depleted mantle source, which was probably modified by subducted materials. The Majiari ophiolite probably formed in a typical back-arc basin above a supra-subduction zone (SSZ) mantle wedge. Intra-oceanic subduction occurred during the Middle Jurassic and collision of the Lhasa and South Qiangtang terranes likely occurred in the Early Cretaceous. Thus, closure of the Bangong-Nujiang Tethys Ocean likely occurred before the Early Cretaceous.